What is often the cause of rust-colored sputum?

Rust-colored sputum is typically associated with a bacterial infection, particularly by Streptococcus pneumoniae. This type of infection can lead to lobar pneumonia, in which blood from inflamed and damaged lung tissues seeps into the airways. The presence of this blood gives the sputum a distinct rusty hue, serving as a hallmark of a pneumococcal infection.

In chronic bronchitis, sputum is usually more clear, white, or even greenish and mucoid rather than rust-colored. Aspiration pneumonia often presents with foul-smelling sputum, while Pneumocystis pneumonia typically causes a dry cough and scant sputum production. Therefore, the specific characteristics of the sputum color and its association with Streptococcus pneumoniae make it the most suitable answer.